The coefficient of x301 in (1+x)500+x(1+x)499+x2(1+x)498+...+ x500 is [2023]
(2)
The coefficient of x301 in
(1+x)500+x(1+x)499+x2(1+x)498+…+x500
=C301500+C300499+C299498+…+C0199
=C199500+C199499+C199498+…+C199199=C200501